1. Field of the Invention
The present invention relates to a flat wire manufacturing method of manufacturing a flat wire having high dimensional accuracy for forming a ring gear by processing a round rod by cold working without requiring tempering to soften the surface of the flat wire hardened by cold working.
2. Description of the Related Art
There are various flat wire manufacturing methods of manufacturing flat wires for forming ring gears and spiral wires. Those methods include a flat wire manufacturing method of manufacturing a flat wire by die drawing a hot-rolled flat wire, a flat wire manufacturing method of manufacturing a flat wire by die drawing a hot-rolled round rod, a flat wire manufacturing method of manufacturing a flat wire by cold-drawing a hot-rolled round rod and a flat wire manufacturing method of manufacturing a flat wire by hot-rolling a hot-rolled round rod.
Although the flat wire manufacturing method that produces a flat wire by processing a round rod only by a cold-rolling process or a hot-rolling process can produce the flat wire at a high productivity because the round rod can be rolled at a high rolling speed, the flat wire manufacturing method cannot produce a flat wire having a high dimensional accuracy. Flat wires produced by a hot rolling process are inferior in dimensional accuracy to those produced by a cold rolling process and need to be processed by machining processes to remove scales and to a decarburized layer. When a flat wire is produced by processing a round rod by a die drawing process, the round rod cannot fill up a drawing die 5 as shown in FIG. 6A unless the diameter of the round rod is greater than the width of the die opening of the drawing die 5. Therefore, a round rod 1 of a very large diameter as shown in FIG. 6B is needed to produce a flat wire having a high flatness. The area reduction of the round rod 1 having such a large diameter is inevitably large and the round rod 1 is broken during die drawing.
When a wide flat wire is produce by processing a round rod at a high working ratio by a cold working process, cracks are liable to be produced in the side surfaces of the flat wire. A method of manufacturing a flat wire for forming a spiral spring disclosed in JP-A 64-27703 processes side parts of the flat wire by an area reducing process to reduce the area by an area reduction in the area reduction range between 1.5 and 15% in the direction of the width of the flat wire at least once in an initial stage of cold rolling process.
The inventors of the present invention examined the area reduction range between 1.5 and 15% for the side parts of the flat wire in the direction of the width mentioned in JP-A 64-27703 through experiments. It was found that the area reduction range between 1.5 and 15% does not have direct relation with the desired hardness of a cold-drawn flat wire for a ring gear and the hardness of the flat wire for a ring wire is dependent on the total reduction of area in the cold drawing process. It was also found that the flat wire finished only by the cold drawing process mentioned in JP-A 64-27703 has low dimensional accuracy, has major surfaces and side surfaces respectively having different hardnesses, and is unsatisfactory in quality.